| Vitrification is considered as an attractive procedure for municipal solid waste incineration ash, which can realize the dramatic reduction of incineration ash volume, high immobilization of heavy metal and destruction of hazardous organics. This disposal method avoids the environmental pollution caused by the leaching of heavy metal and harmful elements. The main oxide components of molten ash slag are CaO, Al2O3, SiO2, Fe2O3, MgO, and Na2O, accounting for more than90%of the total mass. The thermodynamic properties of CaO-MgO-Al2O3-SiO2-FeOx-Na2O system are significant for the harmless disposal of incineration and utilization of molten ash slag, which are the basic thermodynamic data.A study on the thermodynamic properties of the CaO-MgO-Al2O3-SiO2-FeOx-Na2O system was carried out by using CALPHAD method in this paper. The self-bulit database of CaO-MgO-SiO2-FeOx quarternary oxide system has been developed, and the activity of FeO in CaO-SiO2-FeOx liquid system at1673K, phase diagram of MgO-SiO2-FeOx, CaO-SiO2-FeOx as well as CaO-MgO-SiO2-FeOx system for different temperatures and oxygen partial pressures were calculated based on the self-built thermodynamic database. The conclusions are as follows.(1) Based on the FactSage software, the optimized parameters of thermodynamic properties of CaO-SiO2-FeOx and MgO-SiO2-FeOx systems were obtained, and a thermodynamic database of CaO-MgO-SiO2-FeOx quaternary system was built.(2) The FeO activity of liquid phase in the CaO-SiO2-FeOx oxide system at1673K was calculated by self-built database. The calculated activity of FeO is a little smaller compared with the literature data, however, the two results showed the same tendency with the increase of basicity. When the mole ratio of CaO and SiO2was92/8, there was big difference between the calculated results and literature data. (3) The calculated phase diagram of MgO-SiO2-FeOx system showed that, the liquid phase emerged near the CaO-SiO2borderline in the FeOx-rich region when the partial pressure of oxygen was10-8atm. With the increase of partial pressure of oxygen to0.21atm, spinel solid solution emerged, which changed the phase relationship in FeOx-rich area, and the liquid phase region moved to the poor-FeOx area and shrinked. For solid solution region, the calculated resule by self-built database and FACT database was very similar. While for the liquid phase region, calculation results of selt-built database was a little smaller than the result of FACT database.(4) The phase diagram of CaO-SiO2-FeOx calculated by selt-built database was in agreement with that calculated by FACT database for1873K and10-8atm oxygen partial pressure. Comparing the calculated phase diagrams of CaO-MgO-SiO2-FeOx system with7.5wt%MgO, MgO solid phase emerged in the phase diagram by FACT database, while no MgO solid phase can be found in the phase diagram by self-built database.(5) The self-built database can be considered as one of the basic databases to predict the thermodynamic properties of CaO-MgO-Al2O3-SiO2-FeOx-Na2O system. |
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